• 基本代碼

class Parent extends StatefulWidget {
  const Parent({Key? key}) : super(key: key);

  @override
  State<Parent> createState() {
    debugPrint('parent widget----------createState');
    return _ParentState();
  }
}

class _ParentState extends State<Parent> {
  bool inContainer = false;

  @override
  void initState() {
    debugPrint('parent state----------initState');
    super.initState();

    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      debugPrint('parent state----------addPostFrameCallback');
    });
  }

  @override
  void didChangeDependencies() {
    debugPrint('parent state----------didChangeDependencies');
    super.didChangeDependencies();
  }

  @override
  Widget build(BuildContext context) {
    debugPrint('parent state----------build');
    return Scaffold(
      appBar: AppBar(
        title: const Text('one child'),
      ),
      body: Column(
        children: [
          ElevatedButton(
            onPressed: () {
              setState(() {
                inContainer = !inContainer;
              });
            },
            child: const Text('change'),
          ),
          const Child()
        ],
      ),
    );
  }

  @override
  void didUpdateWidget(covariant Parent oldWidget) {
    debugPrint('parent state----------didUpdateWidget');
    super.didUpdateWidget(oldWidget);
  }

  @override
  void activate() {
    debugPrint('parent state----------activate');
    super.activate();
  }

  @override
  void deactivate() {
    debugPrint('parent state----------deactivate');
    super.deactivate();
  }

  @override
  void dispose() {
    debugPrint('parent state----------dispose');
    super.dispose();
  }
}

class Child extends StatefulWidget {
  final v = 1;
  const Child({Key? key}) : super(key: key);

  @override
  State<Child> createState() {
    debugPrint('child widget----------createState');
    return _ChildState();
  }
}

class _ChildState extends State<Child> {
  @override
  void initState() {
    debugPrint('child state----------initState');
    super.initState();

    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      debugPrint('child state----------addPostFrameCallback');
    });
  }

  @override
  void didChangeDependencies() {
    debugPrint('child state----------didChangeDependencies');
    super.didChangeDependencies();
  }

  @override
  Widget build(BuildContext context) {
    debugPrint('child state----------build');
    return const Text('cus-widget');
  }

  @override
  void didUpdateWidget(covariant Child oldWidget) {
    debugPrint('child state----------didUpdateWidget');
    super.didUpdateWidget(oldWidget);
  }

  @override
  void activate() {
    debugPrint('child state----------activate');
    super.activate();
  }

  @override
  void deactivate() {
    debugPrint('child state----------deactivate');
    super.deactivate();
  }

  @override
  void dispose() {
    debugPrint('child state----------dispose');
    super.dispose();
  }
}

初始渲染后的打印：

flutter: parent widget----------createState
flutter: parent state----------initState
flutter: parent state----------didChangeDependencies
flutter: parent state----------build
flutter: child widget----------createState
flutter: child state----------initState
flutter: child state----------didChangeDependencies
flutter: child state----------build
flutter: parent state----------addPostFrameCallback
flutter: child state----------addPostFrameCallback

點(diǎn)擊change按鈕打印：

flutter: parent state----------build

修改Child的v變量的值之后熱更新：

flutter: parent state----------build
flutter: child state----------didUpdateWidget
flutter: child state----------build

修改Child在Column中的順序液样，或者不改變層級使用LocalKey也會調(diào)用didUpdateWidget。因?yàn)?code>Widget本身不可變，當(dāng)你修改了Widget的屬性之后或者直接rebuild不可避免需要重新創(chuàng)建一個(gè)新的Widget替換舊的于宙，而同時(shí)舊的State可以復(fù)用，就會調(diào)用State的didUpdateWidget悍汛。

• 關(guān)于deactivate

將Parent的build方法修改如下：

Widget build(BuildContext context) {
    debugPrint('parent state----------build');
    return Scaffold(
      appBar: AppBar(
        title: const Text('one child'),
      ),
      body: Column(
        children: [
          ElevatedButton(
            onPressed: () {
              setState(() {
                inContainer = !inContainer;
              });
            },
            child: const Text('change'),
          ),
          inContainer
              ? Container(
                  color: Colors.blue,
                  child: const Child(key: ValueKey('123')),
                )
              : const Child(key: ValueKey('123'))
        ],
      ),
    );
  }

點(diǎn)擊change按鈕

flutter: parent state----------build
flutter: child widget----------createState
flutter: child state----------initState
flutter: child state----------didChangeDependencies
flutter: child state----------build
flutter: child state----------deactivate
flutter: child state----------dispose
flutter: child state----------addPostFrameCallback

再次點(diǎn)擊change按鈕

flutter: parent state----------build
flutter: child state----------deactivate
flutter: child widget----------createState
flutter: child state----------initState
flutter: child state----------didChangeDependencies
flutter: child state----------build
flutter: child state----------dispose
flutter: child state----------addPostFrameCallback

需要注意捞魁，兩次打印deactivate位置變了，由于兩次點(diǎn)擊ChildState的層級不同离咐，優(yōu)先處理更深層次谱俭，再處理父層。

盡管有Key宵蛀，但是由于層級改變了昆著，所有不會復(fù)用State

• 關(guān)于activate

將Child的build修改如下：

  final GlobalKey _gk = GlobalKey();

  @override
  Widget build(BuildContext context) {
    debugPrint('parent state----------build');
    return Scaffold(
      appBar: AppBar(
        title: const Text('one child'),
      ),
      body: Column(
        children: [
          ElevatedButton(
            onPressed: () {
              setState(() {
                inContainer = !inContainer;
              });
            },
            child: const Text('change'),
          ),
          inContainer
              ? Container(
                  color: Colors.blue,
                  child: Child(key: _gk),
                )
              : Child(
                  key: _gk,
                )
        ],
      ),
    );
  }

點(diǎn)擊change按鈕，打印如下：

flutter: parent state----------build
flutter: child state----------deactivate
flutter: child state----------activate
flutter: child state----------didUpdateWidget
flutter: child state----------build

這里可以看到activate的打印术陶，因?yàn)槭褂昧?code>GlobalKey凑懂，且改變了Child的層級，其State在rebuild過程中從樹中移除后又重新插入新的位置梧宫。

• 關(guān)于didChangeDependencies

可以看到初始化渲染調(diào)用了child-didChangeDependencies接谨，這是因?yàn)樽铋_始child并不存在摆碉，在parent動(dòng)用updateChild的時(shí)候不存在舊的child，那么會直接調(diào)用inflateWidget->mount->_firstBuild->didChangeDependencies疤坝。

在inflateWidget中兆解，會判斷‘GlobalKey’，即能不能在inactiveElement之中找到可用的Element跑揉，如果找到了就不用重新創(chuàng)建新的Element锅睛，也就不會調(diào)用didChangeDependencies。

如果沒找到可用的Element历谍，就會創(chuàng)建新的Element现拒，這個(gè)時(shí)候就會調(diào)用mount方法，將新的Element插入樹中望侈，注意由于Element.mount不調(diào)用_firstBuild印蔬，只有ComponentElement他重寫了mount，其中調(diào)用了_firstBuild脱衙。之所以會調(diào)用ComponentElement.mount是因?yàn)?code>StatefulElement繼承自ComponentElement侥猬，而我們demo中的Child是StatefulWidget，對應(yīng)的Element就是StatefulElement捐韩。

在大部分情況下退唠，調(diào)用didChangeDependencies的時(shí)候都會調(diào)用initState，那么在什么時(shí)候才只會調(diào)用didChangeDependencies荤胁？在使用InheritedWidget的時(shí)候瞧预，代碼如下：

class ShareDataWidget extends InheritedWidget {
  final int data;

  const ShareDataWidget(
    this.data, {
    Key? key,
    required Widget child,
  }) : super(key: key, child: child);

  static ShareDataWidget? of(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType<ShareDataWidget>();
  }

  @override
  bool updateShouldNotify(covariant ShareDataWidget oldWidget) {
    return oldWidget.data != data;
  }
}

將Parent的build修改如下：

@override
  Widget build(BuildContext context) {
    debugPrint('parent state----------build');
    return Scaffold(
      appBar: AppBar(
        title: const Text('one child'),
      ),
      body: Column(
        children: [
          ElevatedButton(
            onPressed: () {
              setState(() {
                inContainer = !inContainer;
              });
            },
            child: const Text('change'),
          ),
          ShareDataWidget(
            inContainer ? 1 : 2,
            child: const Child(),
          )
        ],
      ),
    );
  }

這個(gè)時(shí)候點(diǎn)擊change按鈕打印如下：

flutter: parent state----------build
flutter: child state----------didChangeDependencies
flutter: child state----------build

當(dāng)ShareDataWidget只是一個(gè)普通Widget的時(shí)候，打咏稣：

flutter: parent state----------build

這里已經(jīng)很清晰了垢油，Parent每次build其實(shí)是重新運(yùn)行了build函數(shù)內(nèi)的代碼，那么其中的Widget必定會改變圆丹，而對應(yīng)的Element和State則盡量不改變滩愁。那么如果沒有特殊的變化，單純rebuild是不會觸發(fā)除build之外的任何生命周期函數(shù)运褪。而如果使用了InheritedWidget惊楼，那么當(dāng)其攜帶的信息改變時(shí)就有必要通知依賴它的State，這里就是通過didChangeDependencies告知這種變化秸讹。